The present invention relates to holographic elements and to a method and apparatus for the use of such holographic optical elements.
Narrow bandwidth optical filters employing reflective type holographic optical elements are known, and it is also known that the minimum obtainable bandwidth of a holographic optical element decreases as the holographic material, or film, thickness increases. Thus, it is clear that to attain very narrow bandwidth optical filters, say of the order of one nanometer spectral bandwidth, relatively thick films are required.
However, construction of suitable thick reflective type holographic optical elements with sufficient effective thickness for this purpose has hitherto been difficult to achieve. The effective thickness is primarily limited by a) the ability of any necessary processing solutions to diffuse into the depth of the film, b) attenuation of the exposure beam through the depth of the film, and c) the dependence of the average film refractive index on exposure causing a progressive dephasing of the recorded interference fringes through the depth of the film.
As a consequence of these problems, some of the useable thickness of the film is effectively wasted, and the recorded hologram is highly variable or "chirped". Thus, it is not possible to achieve very narrow operating bandwidths which would otherwise be attainable with such thick films.
Generally, it can be said that very thick films are more difficult to produce as compared with thinner films, and the production of thick films is also prone to giving a low yield during the coating and preparation stage because of film adhesion and quality problems.
An aim of this invention is to provide a method and apparatus for the use of holographic optical elements which overcomes the aforementioned problems in an efficient manner.